The invention relates to an apparatus for the return of lubricant for a refrigeration machine and also to a refrigeration machine with an apparatus of this kind.
Refrigeration plants with refrigerants are usually operated with closed refrigerant circuits, in which a compressor sucks in gaseous refrigerant directly or indirectly from a vaporizer, compresses it and passes it on to a condenser, in which the compressed refrigerant is liquefied while giving up heat. The liquid refrigerant is then relaxed almost to vaporization pressure via a restrictor, in order to subsequently transfer back to the gaseous state in the vaporizer while taking up heat of vaporization.
Depending on the amount to be forwarded, volumetrically forwarding compressors or centrifugal compressors, for example, can be employed as the compressor and in each case friction losses can occur at bearings and dynamic sealing locations among other things. Refrigeration plants, which use carbon dioxide (CO2) as a refrigerant for example, have almost always been operated with oil-free compressors in the past, which means without corresponding lubricant. Furthermore, lubricated compressors are also known which, for example, use soluble ester oil as a lubricant. The soluble ester oil can be returned via a thermo-syphon pump, as is known in the case of FCKW refrigerants, for example.
However, non-soluble oils and lubricants are often better suited for the refrigerant circuit.
Thus it is known for example to operate refrigeration plants, which use ammonia as a refrigerant, with various mineral oils, which are practically insoluble in ammonia. These mineral oils display a high viscosity as a rule at low temperatures, which has the result that the separated out mineral oil, which is carried along in the stream of ammonia from the compressor, has to be removed from the refrigerant again via complicated oil separators, since it settles on cold wetted surfaces, impairs the heat transfer in the heat exchangers, has a disturbing effect in valves, regulating units and the like and, finally, has the tendency to settle out as a sump in the vaporizer.
Non-soluble oils such as PAO oils for example, which are lighter than the liquid refrigerant as a rule, are better suited as a lubricant than the liquid refrigerant CO2. This kind of lubricant obviously also causes the problems which have been outlined in detail above and thus has to be continually removed from the refrigerant circuit.